1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
The adoption of electrophotographic image forming apparatuses as copying machines, printers, facsimile machines, and the like equipment has been widespread. In general, image formation is carried out by an electrophotographic image forming apparatus as follows. As a photoreceptor, the one having formed on its surface a photosensitive layer containing a photoconductive substance is used. After electric charge is applied to the surface of the photoreceptor in such a way that the entire surface is charged evenly, an electrostatic latent image corresponding to specific image data is formed thereon through various process steps for image formation. The electrostatic latent image is developed into a toner image by using a developing agent containing toner that has been supplied from developing means. The toner image is directly transferred onto a recording medium such as a paper sheet, or is transferred onto an intermediary transfer medium once and is thereafter transferred onto a recording medium. Lastly, the toner image transferred onto the recording medium is fixed into place. The fixation of the toner image onto the recording medium can usually be achieved by heating and pressurizing the recording medium with use of fixing means constructed based on a heat fixing method, for example, a fixing roller comprising heating means. However, there has been a growing trend in recent years to achieve energy conservation as a countermeasure against global warming. As a natural consequence thereof, a reduction in power consumption required to fix a toner image onto a recording medium has come to be increasingly demanded of an electrophotographic image forming apparatus. Unfortunately, the heat fixing method presents the following disadvantages. Firstly, heating means is used inside the apparatus, wherefore the apparatus is interiorly heated to a high temperature. This creates the need to enhance the heat resistance of constituent components, which results in an undesirable increase in material costs. Secondly, image fixation cannot be effected until a part to be fixed has been heated to a predetermined temperature. In this case, the time required for the to-be-fixed part to reach the predetermined temperature, namely, warm-up time, tends to be increased. Thirdly, much time heeds to be spent in fixing a multi-color toner image onto a recording medium as compared to the case of fixing a monochromatic toner image. Accordingly, a reduction in time required to fix a multi-color toner image has been sought after.
In view of the foregoing requirements, there has been proposed a wet fixing method that employs a fixing fluid containing water and a liquid which is dissoluble or dispersible in water and produces the effect of softening or swelling out toner. According to the wet fixing method, a toner image is brought into a softened or swollen state through the application of the fixing fluid. In this state, the toner image is adhered to a recording medium, and is then fixed into place by pressurization. The wet fixing method consumes far less power than does the heat fixing method, and is thus of a useful method from energy-saving standpoint. Another advantage is that the wet fixing method does not require a large amount of heat to fix a multi-color toner image. This, in contrast to the case of adopting the heat fixing method, enables a reduction in fixation time. Accordingly, various improvements have been made to date to make practical use of the wet fixing method. For example, an image forming apparatus composed of preliminary fixing means, fixing fluid applying means, and fixing means has been proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 2004-294847). Specifically, in the preliminary fixing means, a toner image transferred onto a recording sheet is subjected to pressurization to augment the adhesion of the toner image to the recording sheet. In the fixing fluid applying means, a fixing fluid is applied only to a certain region on the recording sheet where the toner image is adhered by means of an ink jet head, a mist producing head, or otherwise. In this way, the toner can be softened or swollen. In the fixing means, the toner image, now kept in a softened or swollen state by the application of the fixing fluid, is fixed onto the recording sheet through application of heat and pressure. According to the techniques disclosed in JP-A 2004-294847, the toner image is softened or swollen by the application of the fixing fluid prior to the final fixing operation conducted by the fixing means. This makes it possible to set a heating temperature for the fixing means to fall in a range of from approximately 70° C. to 100° C. This temperature level is lower than the level of a normal thermal fixing temperature. Moreover, in a case where the percentage of print coverage on an image to be formed is low, no heating operation is carried out to reduce power consumption. In this image forming apparatus, however, the fixing fluid is directly applied only to the region on the recording sheet where the toner image is adhered. Note that this toner image bearing region is not covered with toner completely but has a part to which no toner is adhered. The fixing fluid applied to this toner-absent part finds its way into the recording sheet swiftly, which is causative of generation of wrinkles, curls, or the like in the recording sheet. Inconveniently, the generation of wrinkles, curls, or the like is particularly notable in this image forming apparatus, because the water content in the fixing fluid cannot be vaporized readily due to the relatively low fixing temperature set for the fixing means.
Moreover, an image forming apparatus composed of toner image forming means, an intermediary transfer belt, a roller-shaped applying member, and transferring and fixing means is known. The intermediary transfer belt is driven to rotate while bearing on its surface a toner image formed by the toner image forming means. The roller-shaped applying member acts to apply a fixing fluid to an unfixed toner image formed on the intermediary transfer belt in a contact manner. The transferring and fixing means acts to transfer and fix the toner image borne on the intermediary transfer belt onto a recording sheet. In this construction, the fixing fluid is applied to the toner image borne on the intermediary transfer belt in a contact manner, so that the toner constituting the toner image can be swollen and softened. In this state, the toner image is transferred and fixed onto the recording sheet. In this image forming apparatus, just as is the case with the image forming apparatus disclosed in JP-A 2004-294847, in order to achieve a reduction in power consumption, it is absolutely necessary to keep low the heating temperature for the application of the fixing fluid and for the image transference and fixation. Accordingly, this image forming apparatus failed to overcome the drawback associated with the wet fixing method, namely, the generation of wrinkles, curls, or the like in the recording sheet that occurs when the fixing fluid applied to the toner-absent part of the toner image borne on the intermediary transfer belt finds its way into the recording sheet in accompaniment with the image transference and fixation. There is also known an image forming apparatus composed of toner image forming means, an intermediary transfer belt, heating means for applying heat to the intermediary transfer belt, a roller-shaped applying member, and transferring and fixing means. In this construction, a fixing fluid is applied to a toner image borne on the intermediary transfer belt kept in a heated state in a contact manner, so that the toner constituting the toner image can be swollen and softened. In addition to that, the water content in the fixing fluid applied to a toner-absent part is vaporized. This helps prevent wrinkles, curls, or the like from occurring in a recording sheet at the time of carrying out image transference and fixation thereon. In this image forming apparatus, however, the applying member is kept in constant contact with the intermediary transfer belt in a heated state. Furthermore, a customarily-used fixing fluid is highly volatile. Therefore, the amount of the fixing fluid on the surface of the applying member becomes uneven, which leads to a failure of applying the fixing fluid to the toner image evenly. As a result, there arise lack of uniformity in the application of the fixing fluid and thus irregularity in an image obtained.
Moreover, a fixing apparatus composed of a fixing fluid chamber for storing a fixing fluid, a applying member formed in the shape of roller, and shutter means has been proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 2004-333866 (see FIG. 7 in particular)). The fixing fluid chamber has an opening formed in the perpendicularly lower part thereof. The applying member is disposed in the fixing fluid chamber interiorly thereof, part of which protrudes downwardly from the opening so as to stop up the opening. The applying member is immersed in the fixing fluid, is rotatably supported, and has formed on its surface grooves for retaining the fixing fluid thereon. The shutter means allows the closing and opening of that exposed part of the applying member which protrudes downwardly from the opening. In this fixing apparatus, when the application of the fixing fluid is needed, the exposed part of the applying member is opened by actuating the shutter means, whereupon the fixing fluid can be applied to a toner image bearing recording sheet arranged below the fixing apparatus in a contact manner by the applying member. On the other hand, when the application of the fixing fluid is not needed, the exposed part of the applying member is closed by actuating the shutter means. Moreover, since an excess of the fixing fluid remaining on the surface of the applying member is removed by a contact action between the applying member and the end of the opening of the fixing fluid chamber, it follows that the fixing fluid exists only in the grooves created on the surface of the applying member. This helps prevent a larger amount of the fixing fluid than is necessary from being applied to the recording sheet. In this fixing apparatus, upon the shutter means being closed, the shutter means and the applying member are brought into contact with each other, wherefore the applying member is no longer rotated. The resumption of rotation of the applying member is effected immediately before the fixing fluid is applied to the recording sheet conveyed to a certain position therebelow, with the shutter means opened to the ready state. In this case, the fixing fluid cannot be spread evenly across the groove on the surface of the applying member, in consequence whereof there results lack of uniformity in the application of the fixing fluid to the recoding sheet. This could lead not only to image irregularity but also to image defects resulting from improper fixation. Another problem is that the grooves created on the applying member are made uniform in dimension, and thus the amount of the fixing fluid to be applied remains invariant. Therefore, in a case where the percentage of print coverage on an image to be formed is varied, depending upon the value of the percentage, the amount of the fixing fluid to be applied is so small that fixation ends in failure, or is so large that the occurrence of image flow and generation of wrinkles, curls, or the like induced by the fixing fluid are inevitable. For example, even if the number of rotation of the applying member is so controlled as to cope with a change in print coverage rate, since the applying member and the end of the opening of the fixing fluid chamber are kept in sliding contact with each other, this construction presents a problem in the provision of long-term durability. Moreover, whenever the shutter means is opened or closed, the applying member makes contact therewith. At this time, the surface of the applying member may suffer from scratches, which gives rise to lack of uniformity in the application of the fixing fluid. Further, when the fixing fluid is adhered to the surface of the shutter member contacted by the applying member, the fixing fluid may be scattered inside the image forming apparatus in accompaniment with the opening of the shutter member. Besides, in this construction, since the shutter member is so disposed as to cover the applying member from underneath, the fixing fluid may leak from the gap between the shutter member and the fixing fluid chamber or the applying member. In this case, the image forming apparatus may be interiorly contaminated with the fixing fluid leakage. In addition, JP-A 2004-294847 makes no specific suggestion about means for opening and closing the shutter means, the configuration of the lengthwise end of the shutter means corresponding to the lengthwise ends of the fixing fluid chamber and the applying member, and so forth. This makes it difficult to implement the invention disclosed in JP-A 2004-294847.